Tear-open bag and three-layer coextrusion film for making the bag

ABSTRACT

A tear-open packaging is formed by a pair of plastic film panels having inner faces joined together by a weld. One of the film panels is formed by a two- or three-layer coextrusion having a nonsticky foam core layer and a weldable inner layer that is secured by the weld directly to the inner face of the other film panel. The strength of the core layer and of the bond between the core layer and inner and outer layers are such that, when the two panels are pulled away from each other, the core layer or an interface between the core layer and the weldable inner layer will tear along the weld. If the weld does not extend directly to an edge of the film tear-open packaging, the weldable layer breaks on both sides of the weld.

FIELD OF THE INVENTION

The present invention relates to a tear-open packaging. More particularly this invention concerns a three-layer coextrusion film for making a tear-open bag.

BACKGROUND OF THE INVENTION

A film tear-open packaging comprising a first panel from a flexible coextrusion film and a second film panel. The first film panel has an at least two-layer coextruded structure with an inner layer from a nonsticky material and an outer layer. The term “nonsticky” means the property of not sticking to something else, for instance like a pressure-sensitive adhesive as in EP 0 978 460. The inner face formed by the inner layer of this first film panel is separably connected to the second film panel along a weld.

The invention thus also relates to a simple film tear-open packaging which, for a first use, can be opened by simply pulling apart the two film panels, for example a front wall and a rear wall, without the need of additional auxiliary means and without cutting off a top edge, corner or the like. At least the coextrusion film forming the first film panel is not intended for providing a reclosing function through adhesive or the like. The provision of additional mechanisms in the form of a zipper closure or a slider closure is not categorically excluded. However, the present invention relates in particular to a simple, cost-effective film tear-open packaging that has no reclosing properties.

The film tear-open packaging can in particular involve a tray packaging with a cover film or also a bag packaging. Corresponding tray packagings are used, for example, for packaged food such as meat, sausage, cheese or the like. Bags are also preferably used for foodstuffs such as, for example, potato chips. In contrast to tear-open packagings, of which a section is torn off, this has the advantage that no scraps are produced that need to be disposed of separately, and the film tear-open packaging also approximately maintains its shape after being opened.

Generic film tear-open packagings are known from practice. In order to enable simple handling of such a film tear-open packaging, opening, on the one hand, should be possible using an opening force as low as possible while, however, is unintentional separation of the film panels juxtaposed at the weld is to be avoided. Moreover, the two film panels also have to be welded together in such a manner that when making the film tear-open packaging, a reliable tight seal is ensured. Finally, film tear-open packagings are disposable packagings so that production costs have to be a low as possible, which is why preferably simple plastics such as polyolefins shall be used.

In the case of the known film tear-open packagings without reclosing properties, there are different kinds of separation of the connected film panels at the weld when tearing them open.

It is possible to use a weldable layer that causes layer separation within the weldable layer when stretched. Such a separation is also designated as a cohesion fracture or peel. In order to enable such a cohesion fracture at tensile forces that are not excessively high, a polyethylene weldable layer may include an admixture of polybutylene. Also, an additive can be added that is not miscible with the basic substance of the weldable layer so as to reduce the strength of the weldable layer itself.

As an alternative the weld itself is torn open when the interconnected film panels are pulled away from one another. The interface between the layers that are welded together is then also at the same time the parting plane so that an adhesion fracture or peel occurs. An adhesion fracture can be controlled by using polymers that have only limited mutual adhesion, to which end copolymers for example can be used. However, it can be a disadvantage that reliable closing cannot be guaranteed under all circumstances during sealing, so the material selection is also greatly limited.

In one variant of adhesion fracture, layer separation is not in one of the layers that are welded together, but between one of the weldable layers and an adjoining layer covering the weld, which requires that the weldable layer has to be torn through perpendicular the film plane on both sides of the weld. Such a separation is also designated as a tearing peel.

With regard to the designation of different fracture patterns, reference is made to DIN EN ISO 10365.

In the case of a reclosable film tear-open packaging, which is of comparable cost and difficulty to produce, it is known from above-cited EP 0 978 460 to use a core layer from a permanent or pressure-sensitive adhesive behind an external weldable layer, so that, when tearing open, the weldable layer fractures perpendicular to its plane laterally from a weld so as to expose the pressure-sensitive permanent adhesive of the core layer. With regard to the core layer, this may result in a cohesion fracture or an adhesion fracture. Such reclosable packagings are also known from FR 2 686 578.

JP 2002-154555 discloses a film tear-open packaging that has a nonoriented weldable layer that is laminated with a foamed oriented film layer. In order to obtain an adhesion fracture with respect to the adjoining foamed film layer when opening the packaging, aluminum metallization is provided between the foamed oriented layer and the nonoriented weldable layer. Due to the metallization and the different orientation of the layers that are connected to one another, the manufacturing method is complex. Simple coextrusion films cannot be used.

U.S. Pat. No. 5,882,782 describes a web-shaped laminate from thermoplastic foam and a molded body produced therefrom. For example, a stiff foam tray is provided that subsequently can be separated completely from an adjoining film so as to allow separate disposal.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide an improved tear-open packaging and three-layer coextrusion film for making it.

Another object is the provision of such an improved tear-open packaging and three-layer coextrusion film for making it that overcomes the above-given disadvantages, in particular that is configured in the simplest possible manner and is characterized by good tear-open properties.

SUMMARY OF THE INVENTION

A tear-open packaging is formed by a pair of plastic film panels having inner faces joined together by a weld. One of the film panels is formed by a two- or three-layer coextrusion having a nonstick foam core layer and a weldable inner layer. The weldable inner layer is turned toward the other film and secured by the weld directly to the inner face of the other film panel. The strength of the core layer and of the bond between the core layer and inner and outer layers are such that, when the two panels are pulled away from each other, the core layer or an interface between the core layer and the weldable inner layer will tear along the weld. If the weld does not extend directly to an edge of the film tear-open packaging, the weldable layer breaks on both sides of the weld, namely substantially is perpendicular to the plane of the coextrusion film.

If, according to an alternative solution, the coextrusion film is formed from at least two layers and the first layer of the coextrusion film is directly sealed to the second film panel, according to the invention the first layer is foam, and the strength of the weld and the bond strength of the layers of the coextrusion film are set in such a manner that when separating the film panels at the weld, layer separation takes place within the first layer or at a interface of the first layer to the outer layer or laminated layer. If the layer separation takes place at the interface to the outer layer or laminated layer, the foam first layer tears open up to this interface. Separating normally does not take place directly at the molten material of the weld, and in this case the weakening by foaming would have no impact.

According to the invention, the internal strength and also the bond strength of the core layer or the first layer is reduced with respect to the adjoining layers. This reduction is implemented according to the invention by foaming so that adding further raw materials can be avoided, which is advantageous for a plurality of reasons. First, the film tear-open packaging can be produced completely or at least largely from polyolefin, in particular from polyethylene, and then the film tear-open packaging as a whole can easily be recycled. Also, eliminating additional raw materials is advantageous with regard to processing and costs. Producing the coextrusion film only requires setting up the extruder in such a manner that the core layer can be foam. Apart from that, simple commercially available raw materials can be used.

It is also an advantage that the force necessary for tearing open, the so-called peel force, can be accurately set by the degree of foaming and the selection of materials for the different layers.

According to the invention, layer separation does not take place directly at the interface of the interconnected film panels so that high strength and therefore secure bonding is ensured there by the weld.

In the case of an at least three-layered structure of the coextrusion film, however, the weldable layer is advantageously as brittle as possible so that when tearing it open, it can easily break up to the foam core layer that is arranged therebelow.

According to the invention, the first film panel consists of the described coextrusion film having at least two or three layers, and the second film panel can be formed from a different film material, for example a stiff, deep-drawn tray. However, it is in principle also possible that the first film panel and the second film panel are formed from the same coextrusion film. In particular, the entire tear-open film packaging can be formed completely from the described coextrusion film. With regard to the weld, this then results in a matching structure in both directions, and separating within the core layer or the first layer and/or at a interface of the core layer or the first layer takes place in one of the two interconnected film panels.

The coextrusion layer opposite the second film panel can form an outer layer or a laminated layer. In particular, an outer film can be laminated with this layer, which then forms a laminated layer. For example, the outer film can be provided for giving the film tear-open packaging a high-quality appearance and for enabling printing thereon. For example, outer films made of polyester, in particular polyethylene-terephthalate (PET) or biaxially oriented polypropylene (BOPP) are suitable.

As already explained above, the foam core layer or first layer as provided according to the invention enables that the portion of additional admixtures is reduced, and it is also possible to form largely unmixed coextrusion films from polyolefin, in particular polyethylene. The coextrusion film preferably consists of at least 50% by weight polyolefin, in particular polyethylene.

In particular, the outer or cover layer can contain low density polyethylene (LDPE) including linear low density polyethylene (LLDPE) as the main constituent. Particularly preferred is a mixture of LLDPE and LDPE, which can be provided in a mixture ratio of 70:30, for example. However, other materials are in principle also suitable for the outer or cover layer.

It is also preferred that the core layer or first layer is mainly formed from polyolefin, in particular from low density polyethylene.

By using a foam core layer or first layer, the density thereof is reduced according to the degree of foaming. The density of the core layer or first layer ranges, for example, between 0.1 g/cm³ and 0.8 g/cm³, preferably 0.3 g/cm³ and 0.6 g/cm³, more preferably between 0.4 g/cm³ and 0.5 g/cm³. In the case of conventional materials and in particular for the preferred low density polyethylene (LDPE) or linear low density polyethylene (LLDPE), this is approximately equivalent to halving the density and therefore to doubling the thickness compared to a nonfoam film layer.

Particularly uniform good mechanical properties are achieved if the core layer or first layer has a microcellular foam structure that can be formed by the so-called MuCell method, for example.

During the extrusion or immediately after the melt exits the extrusion gap, a foaming agent is added to the melt, which causes foaming. In particular physical foaming where at the beginning a pressurized foaming agent is added to the melt which then expands upon exiting the nozzle gap of the coextrusion nozzle is employed. Particularly suitable as foaming agents are nitrogen (N₂) and carbon dioxide (CO₂). Prior to adding the foaming agent, it is in the form of a supercritical fluid that combines the incompressibility of a fluid and the solubility of a gas. The foaming agent dissolves in the polymer melt and once distributed therein forms a monophase system with the plastic melt. Due to a rapid pressure drop upon exiting the extrusion nozzle, nucleation sites form in the polymer melt. Gas is released from the melt in a controlled manner thereby forming a very fine uniform foam structure. A corresponding method is described in, among others, U.S. Pat. No. 6,051,174.

In order to forma the smallest possible cells as uniformly as possible, the core layer or first layer may contain an admixture of particles that facilitate cell formation during the extrusion process and therefore are also designated as a nucleation agent. Such an admixture of particles can consist of talcum or can comprise talcum as the main constituent.

In the foam state, the thickness of the core layer can range, for example, between 10 μm and 100 μm, preferably between 50 μm and 70 μm.

The total thickness of the coextrusion film can range between 20 μm and 250 μm, preferably between 60 μm and 120 μm, and the optionally provided weldable layer preferably can have a thickness between 3 μm and 100 μm, more preferably between 8 μm and 30 μm, for example about 15 μm.

The invention is also the use of a flexible, at least three-layered coextrusion film comprising a core layer from a nonsticky foam material that is between a weldable layer and an outer or cover layer for producing a film tear-open packaging, with the requirement that the coextrusion film as the first film panel is separably connected along a weld to a second film panel.

In the course of the claimed use, the above-described film tear-open packaging then becomes apparent.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a section through part of a tear-open bag when sealed;

FIG. 2 is a view like FIG. 1 but with the bag torn open;

FIGS. 3 and 4 are views like respective FIGS. 1 and 2 of another packaging according to the invention;

FIG. 5 is a view like FIG. 1 of a third embodiment of the invention; and

FIG. 6 is a view like FIG. 1 of a part of a bag constructed so that it cannot be readily torn open.

SPECIFIC DESCRIPTION OF THE INVENTION

As seen in FIG. 1 a film tear-open packaging that is not reclosable has a first film panel 1 and a second film panel 2 each made from a flexible, three-layered coextrusion film 3. In this embodiment of the film tear-open packaging, each of the coextrusion films 3 has a weldable inner layer 4, an outer layer 6, and a core layer 5 therebetween. In this embodiment, here is also an outer film 7 that which gives the film tear-open packaging a high-quality appearance and is connected to the outer layer 6. At the same time, the outer film 7 can also be provided as a substrate for printing. Suitable materials for the outer film 7 include, for example, polyethylene-terephthalate (PET) or biaxially oriented polypropylene (BOPP).

FIG. 1 also shows that the two weldable layers 4 of the first film panel 1 and the second film panel 2 are connected to one another at a weld 8, thereby closing the film tear-open packaging. The remaining unillustrated edges of the film tear-open packaging can also be provided with such a weld. However, at least a portion of the edges can also be formed by folding the coextrusion film.

FIG. 2 shows the packaging after tearing open. The weldable layer 4 at the first film panel 1 is torn perpendicular to the surface of the coextrusion film 3 on both sides of the weld 8, and the thickness of the core layer 5 is torn through as a result of the force exerted during tearing open so that a cohesion fracture of the core layer 5 is created.

According to the invention, the core layer is foam and thus formed from a nonsticky material, for instance not with a surface carrying a pressure-sensitive adhesive. By foaming, the strength of the core layer 5 is reduced in a defined manner so that in the illustrated layer structure including the weld 8, the core layer 5 is the weakest point when tearing open, which results in the described cohesion fracture during such tearing open.

Based on this, FIGS. 3 and 4 show an alternative configuration in which only the first film panel 1 is formed from the described coextrusion film 2. In contrast, the second film panel 2′ is formed as a monolithic stiff tray that, at least in the direction of the coextrusion film 3, has a surface that can be welded.

FIG. 4 shows reducing the strength of the core layer by foaming to get lower bonding strength with respect to the adjoining weldable layer 4 so that depending on the materials used, an adhesion fracture is also possible as an alternative to the cohesion fracture illustrated in FIG. 2. Thus the fracture forms, not in the layer 5, but at the interface between the layer 4 and the layer 5

According to the invention, easy tearing open can be achieved with simple means, namely simply by foaming the core layer 5. Thus, it is possible to produce the coextrusion film 3 mainly or exclusively from polyolefin. The weldable layer 4 can consist of polyethylene, in particular low density polyethylene (LDPE), and/or linear low density polyethylene (LLDPE). In order to enable easy tearing of the weldable layer 4 toward the core layer 5 arranged therebelow, in particular, admixtures of polystyrene (PS) or polypropylene (PP) are also useful.

The core layer 5 has a microcellular foam structure with cells filled with nitrogen. The density of the core layer, for example, can range between 0.1 g/cm³ and 0.8 g/cm³, and admixing particles, in particular talcum, is also suitable for improving cell formation. A mixture of LLDPE and LDPE is suitable as the polymeric basis substance of the core layer 6.

A comparable mixture is also particularly preferred for the outer or cover layer 6.

A three-layered coextrusion film 3 comprising a weld 4, a core layer 5 and an outer or cover layer was produced according to the Table 1 as part of a specific example. Nitrogen at a pressure of 250 bar was added as a foaming agent in such an amount that the core layer 5 has twice the thickness with respect to a nonfoam layer from the same material. The density shown in Table 1 refers to the unit g/cm³. The melt flow index MFI (also referred to as MFR) was determined according to DIN EN ISO 1133.

TABLE 1 Weldable layer Core layer Outer or cover layer (15 μm) (60 μm) (15 μm) 70% LDPE (MFI 2-5; 60% LDPE (MFI 2-7; 70% LDPE (MFI 1-2; 190° C./2.16 kg) 190° C./2.16 kg) 190° C./2.16 kg) 30% PS (MFI 15-25; 20% Nucleation 30% LDPE (MFI 1-4; 200° C./5 kg) agent-LDPE-batch 190° C./2.16 kg) (contains talcum) 20% LDPE (MFI 3-9; 190° C./2.16 kg)

FIG. 5 shows an alternative configuration of the invention where the two identical coextrusion films 3′ are each formed from only two layers. Accordingly, the first film panel 1 and the second film panel 2 are connected at inner layers 5′ that accordingly replace the an inner layer and core layers of FIGS. 1-4. In such a configuration of the packaging, foam can also be utilized for enabling easy tearing. Thus, when tearing open directly at the weld, no separation takes place. Rather, tearing open takes place within one of the inner layers 5′, as in the embodiment of FIG. 1, or even between the first layer 5′ and the cover layer 6.

In the illustrated embodiments, the weld 8 is spaced from the outer edge of the film tear-open packaging so that ripping of the coextrusion film 3 takes place along both sides or edges of the weld 8. Of course, the weld 8 can also be extended up the edge of the film tear-open packaging so that then the tear runs in steps toward the outside.

It is often desired that the film tear-open packaging can be easily opened but without allowing that the first film panel 1 can be separated completely from the second film panel 2. In order to achieve this it is possible that, apart from the separable connection, additional nonseparable connections are produced. FIG. 6 shows such a nonseparable connection, where a weld 8′ is produced in certain sections under higher contact pressure and/or a higher sealing temperature. As a result, the core layer 5 is compressed so that the weakening caused by foaming is reduced. Accordingly, the additional weld 8′ produced in certain regions on the film tear-open packaging results in a region that cannot be torn open or is at least difficult to tear open. 

We claim:
 1. In a tear-open packaging formed by a pair of plastic film panels having inner faces joined together by a weld, the improvement wherein: one of the film panels is formed by a three-layer coextrusion having a nonsticky foam core layer sandwiched between an outer layer and a weldable inner layer, the weldable inner layer being turned toward the other film and secured by the weld directly to the inner face of the other film panel, the strength of the core layer and of the bond between the core layer and inner and outer layers being such that, when the two panels are pulled away from each other, the core layer or an interface between the core layer and the weldable inner layer will tear.
 2. The tear-open packaging defined in claim 1, wherein the inner layer is of low-density polyethylene.
 3. The tear-open packaging defined in claim 1, wherein the inner layer has a thickness between 3 μm and 10 μm.
 4. The tear-open packaging defined in claim 1, wherein the other film panel is a foam tray.
 5. In a tear-open packaging formed by a pair of plastic film panels having inner faces joined together by a weld, the improvement wherein: one of the film panels is formed by a two-layer coextrusion having a nonsticky foam layer laminated to an outer cover layer, the foam layer the inner face of the respective panel that is turned toward the other film and secured by the weld directly to the inner face of the other film panel, the strength of the foam layer and of the bond between the foam layer and the outer cover layer and the weld being such that, when the two panels are pulled away from each other, the foam layer or an interface between the foam layer and the weld will tear.
 6. The tear-open packaging defined in claim 5, wherein the other film layer is also formed by a nonsticky foam layer laminated to an outer cover layer and the weld lies between the foam layers.
 7. The tear-open packaging defined in claim 5, further comprising: an outer cover layer on an outer face of the foam layer.
 8. The tear-open packaging defined in claim 5, wherein the coextrusion is at least 50% by weight of polyolefin.
 9. The tear-open packaging defined in claim 8, wherein the polyolefin is polyethylene.
 10. The tear-open packaging defined in claim 5, wherein the outer cover layer is of low-density polyethylene.
 11. The tear-open packaging defined in claim 5, wherein the foam layer is at least 50% by weight polyolefin.
 12. The tear-open packaging defined in claim 5, wherein the foam layer has a density between 0.1 g/cm³ and 0.8 g/cm³.
 13. The tear-open packaging defined in claim 5, wherein the foam layer has cells filled with nitrogen.
 14. The tear-open packaging defined in claim 5, wherein the foam layer includes between 10% by weight and 30% by weight particles.
 15. The tear-open packaging defined in claim wherein the coextrusion has a thickness between 20 μm and 250 μm. 